What is ReCaptcha?

To understand what is reCaptcha we first need to define the term “Captcha”.
Captcha is a test made of an image and distorted text that has to be solved by human users when they want to subscribe to websites that want to make sure the user is a human being and not a bot or a computer (= preventing spam).

Recaptcha is a system that uses Captchas to digitize books text for the same purpose: to ensure the user is not a computer or bot.
Computers can’t navigate sites that are protected by Captchas.

Every day 200 millions of Captchas are being solved. It takes about 10 second to decipher a Captcha but about 150,000 hours of work to accumulate them. This is where reCaptcha comes and help in channeling the big effort spent on Captchas solving into reading books by digitizing process.

How is it done?
By using Optical Character Recognition (OCR) program books text and pages are scanned and transformed into forms that are impossible for computers to decipher. Every word that can’t be read by OCR is placed on an image and displayed together with another word which can be read correctly, that is a “control word”. The user who wants to subscribe to a site that demands verification of human users, is asked to decipher both words. If he solves the “control word” the system assumes the answer is correct for the new one also.

Captchas are taken from internet archives, the New York Times and alike and can be used as protector from mail spam bots as well.